Visual interneurons in the leech brain

Abstract

1. The medicinal leech has five pairs of simple eyes (Fig. 1A). The photoreceptors of some eyes are known to excite one of the bilateral pair of lateral visual (LVa) cells in the first segmentai ganglion (G1). A new pair of neurons, the LVb cells, has been found in subganglion 4 (SubEG4) of the subesophageal ganglion. Since G1 and SubEG4 develop from adjacent ganglionic primordia, the LVa and LVb cells may be serial homologues. 2. The somata and dendrites of the LVa and LVb cells lie in corresponding positions (Fig. 3), and their axons run side-by-side and terminate together in the supraesophageal ganglion. 3. Both neurons have bulbous contralateral branches (Figs. 2, 3), and fibrous ipsilateral dendrites that match the two zones where photoreceptors of the ipsilateral eyes 3–5 have synaptic terminals (Fig. 1C). Moreover, both neurons are Lucifer Yellow dye-coupled to the photoreceptors of those same eyes (Fig. 2B). 4. When the ipsilateral eyes 3–5 are illuminated, the LVa and LVb cells both show a characteristic complex response that includes a compound EPSP produced by electrical coupling potentials from the photoreceptors (Figs. 5, 6), small spike-like events, and conventional-looking action potentials (Fig. 4). 5. The compound EPSP is thought to elicit local spikes in ipsilateral dendritic branches, which then spread electrotonically into the main neurite and soma as small spikes. In turn, the summation of these small spikes in the neurite evidently elicits propagating action potentials. 6. LV cells produce extended plateau-like depolarizations (Fig. 7) in weak or continuously varying light, or in response to current injection. These slow potentials appear to be generated by a noninactivating Na conductance and a TEA-sensitive K conductance (Fig. 8). Since slow potentials are accentuated when extracellular Ca is replaced by Ba (Fig. 9), Ca appears to participate in the response, presumably via a Ca-dependent K channel. 7. Although the LVa and LVb cells may be homologous, they are not identical (Fig. 5). For example, unlike the LVa cells, the LVb cells are not inhibited by input from the contralateral eyes 3–5. Moreover, eyes 1 and 2 weakly inhibit the LVa cells, but weakly excite the LVb cells.